Fluid catalytic cracking (FCC) is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of substantial added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke is deposited on the catalyst. A high temperature regeneration operation within a regeneration zone combusts coke from the catalyst. Coke-containing catalyst, referred to herein as coked catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone.
A common objective of these configurations is maximizing product yield from the reactor while minimizing operating and equipment costs. Optimization of feedstock conversion ordinarily requires essentially complete removal of coke from the catalyst. This essentially complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration or full burn produces a catalyst having less than 0.1 and preferably less than 0.05 wt % coke. In order to obtain complete regeneration, the catalyst has to be in contact with oxygen for sufficient residence time to permit thorough combustion of coke. Partial regeneration occurs when complete regeneration does not occur. Partial regeneration occurs when regeneration produces a catalyst having at least 0.1 and preferably at least 0.05 and typically at least 0.03 wt % coke.
In the regenerator, the coke is burned from the catalyst with oxygen containing gas, usually air. Flue gas formed by burning the coke in the regenerator is treated for removal of particulates and conversion of carbon monoxide, after which the flue gas may be normally discharged into the atmosphere. Conventional regenerators typically include a vessel having a coked catalyst inlet, a regenerated catalyst outlet and a combustion gas distributor for supplying air or other oxygen containing gas to the bed of catalyst that resides in the vessel. Cyclone separators remove catalyst entrained in the flue gas before the gas exits the regenerator vessel. The regenerator includes a dilute phase and a dense phase fluidized catalyst bed disposed in respective upper and lower regions of the vessel.
There are several types of catalyst regenerators in use today. A conventional bubbling bed regenerator typically has just one chamber in which air is bubbled through a dense catalyst bed. Coked catalyst is added, and regenerated catalyst is withdrawn from the same dense catalyst bed. Relatively little catalyst is entrained in the combustion gas exiting the dense bed. Two-stage bubbling beds have two chambers. Coked catalyst is added to a dense bed in an upper, first chamber and is partially regenerated with air. The partially regenerated catalyst is transported to a dense bed in a lower, second chamber and completely regenerated with air. The completely regenerated catalyst is withdrawn from the second chamber.
A combustor-style regenerator or high efficiency regenerator has a lower chamber called a combustor that burns nearly all the coke to CO2 with little or no CO promoter and with low excess oxygen, typically. A portion of the hot regenerated catalyst from the upper regenerator is recirculated to the lower combustor to heat the incoming spent catalyst and to control the combustor catalyst density and temperature for optimum coke combustion rate. As the catalyst and flue gas mixture enters an upper, narrower section of the combustor, the upward velocity is further increased and the two-phase mixture exits through a disengager into an upper chamber. The upper chamber separates the catalyst from the flue gas in the disengager and cyclones and returns the catalyst to a dense catalyst bed which supplies hot regenerated catalyst to both the riser reactor and the lower combustor chamber.
Afterburn is a phenomenon that occurs when hot flue gas that has been separated from regenerated catalyst contains carbon monoxide that combusts to carbon dioxide in a dilute phase of catalyst. Insufficient catalyst is present in the dilute phase to serve as a heat sink to absorb the heat thus subjecting surrounding equipment to higher temperatures that can be over metallurgical limits and perhaps creating an atmosphere conducive to the generation of nitrous oxides that are undesirable for the environment. Incomplete combustion to carbon dioxide can result from insufficient oxygen in the combustion gas, poor fluidization or aeration of the coked catalyst in the regenerator vessel or poor distribution of coked catalyst into the regenerator vessel.
Conventionally, in a partial combustion operation, it is difficult to burn all of the carbon off the catalyst and the residual carbon can have a negative effect on catalyst activity. It is considered to be partial burn in the regenerator when either the oxygen or carbon monoxide content or both of them are present in the flue gas in a concentration of less than 0.1% and typically no greater than 200 ppm respectively at the outlet of the regenerator vessel. To avoid after burn, many refiners add carbon monoxide promoter (CO promoter) metal such as costly platinum to the FCC catalyst to promote the complete combustion to carbon dioxide before separation of the flue gas from the catalyst at the low excess oxygen required to maintain NOx at low levels. While low excess oxygen reduces NOx, the simultaneous use of CO promoter often needed for after burn control can more than offset the NOx advantage of low excess oxygen. The CO promoter decreases CO emissions but increases NOx emissions in the regenerator flue gas.
On the other hand, many refiners use high levels of CO promoter and high levels of excess oxygen to accelerate combustion and reduce afterburning in the regenerator, especially when operating at high throughputs. These practices may increase NOx by up to 10-fold from the 10-30 ppm possible when no platinum CO promoter is used and excess O2 is controlled to below 0.5 vol %.
Therefore, there is a need for improved methods for preventing after burn and generation of nitrous oxides while operating a high efficiency regenerator in a partial burn mode. There is a need for a process and an apparatus to ensure thorough mixing of catalyst and combustion gas in a regenerator that can promote more uniform temperatures and catalyst activity fostering more efficient combustion of coke from catalyst.